1. Condensed Matter Physics & Materials Science Department, Brookhaven National Laboratory, Upton, NY 11973, USA.
2. Laboratory of Atomic and Solid State Physics, Department of Physics, Cornell University, Ithaca, NY 14853, USA.
3. Laboratoire de Physique des Solides, Universite Paris-Sud, 91405 Orsay, France.
4. Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA.
5. School of Physics and Astronomy, University of St Andrews, St Andrews, Fife KY16 9SS, UK.
6. Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, NY 14853, USA.
7. Department of Applied Science and Technology, Politecnico di Torino, 10129 Torino, Italy.
8. Istituto Nazionale di Fisica Nucleare, Sezione di Torino, 10125 Torino, Italy.